In modern telephone installations or systems, usually a plurality of telephony terminals are connected to a central exchange unit, via telecommunication connections which are established between individual telephony terminals or calls coming from outside the internal telephone network to selected telephony terminals. Within the framework of these telecommunication connections, voice data or user data are transmitted to the respective selected telephony terminal from and to the central exchange unit to the respective terminal. The data can be transmitted in the conventional manner or else using an Internet Protocol.
Usually in such telephone installations or systems, in addition to the voice data or user data to and from the respective telephony terminals, further data, namely so-called functional or control data, are transmitted, which are not a direct part of the information exchanged within the communication connection, but form the foundation for carrying out so-called special functions of the telephone system. Such special functions can be, for example, setting-up or control data for activating or deactivating call forwarding operations, functions of answering machines, call charge registration or management information or the like. Alternatively, it is also possible within the framework of such special functions to transmit the telephone number or extension number of the calling party to the respective called telephony terminal and to display it there, so that the user of the called telephony terminal can get information about the calling party already at the moment when the call is coming in. If the called party is absent, telephone numbers of the calling parties can also be stored in a list of telephone numbers or calling parties of the called telephony terminal, within the framework of further special functions, so that the called party after his return to his work place, is informed about the calls that have come in and were not taken by him.
The telephony terminals of the above-mentioned telephone systems are usually provided, in addition to the telephone number keys, with action keys, which carry out preset special functions. For example, firmly stored telephone numbers or telephone numbers which can be stored by the user can be assigned to such action keys. Then, a call on the correspondingly stored telephone number is triggered by actuation of such an action key by a speed dialing key. Further action keys can be provided, for example, for taking a call temporarily and to mute it, to forward a call (“hold” function), to activate or deactivate a function of an answering machine, or the like.
In telephone systems with telephony terminals equipped in such a way, the increasing in special functions and the assignment of corresponding action keys to these special functions may lead to relatively complex and unclear keypads with a multitude of action keys. The large number of such action keys limits the clarity as well as the operatability of the respective telephony terminal.